In various rotating machines, a rotor or rotating member is closely confined within a housing or casing, and it is important that the gap or distance between the casing and the rotating member, referred to as running clearance, be maintained within predetermined limits for safe and effective operation of the machine. One example of such a machine, and one to which this invention is particularly applicable, is a hot gas turbine engine such as an aircraft gas turbine engine. In such an engine, a turbine wheel or rotor having a circumferential row of spaced apart vanes or blades extending therefrom is closely confined within an encircling housing or casing to define a hot gas flow path transversely through the row of blades. Reaction of the blades to the hot gas flow causes rotation of the turbine wheel and appropriate power generation.
In such hot gas turbine engines, the loss of turbine blade reaction from hot gas leakage or bypass through the running clearance space, instead of between turbine blades represents a potential power loss. However, preservation of a minimum clearance gap during engine operation is a necessary precautionary measure to avoid significant rotational contact of the blades with the encircling casing which may lead to failure of engine components as well as the engine as an effective power plant. For these reasons it has become a practice to measure the running clearance of a turbine wheel during its operation and to have a continuous measuring or monitoring system for the running clearance during certain predetermined operations of the turbine. Various operating characteristics of a hot gas turbine engine provide significant difficulties to the use of many known gap and distance measuring devices, particularly those requiring actual contact with a moving member. For example, the environment at the high speed turbine blades is hostile to measuring devices, reaching extreme temperatures in the range of 1200.degree. F. to 1800.degree. F. in the presence of a high temperature corrosive gas stream. This extreme temperature range causes significant differential expansion of various component parts which affects not only any associated measuring means, but also the running clearance gap or distance being measured. Accordingly, measuring devices or systems requiring contact with the rotor or blades have been avoided. With respect to non-contact measuring means, various electrical capacitance systems have been developed to measure the running clearance of hot gas turbine wheels and compressor rotors.
In these prior electrical capacitance systems, a probe member with a sensor end thereon is installed in an appropriate aperture in a rotor housing, for example, so that the sensor end of the probe is exposed to the tips of the turbine blades. The sensor end of the probe adjacent the moving blades is fitted with an electrical capacitor electrode which may be positioned closely adjacent to or at the inner surface of the closely confining casing or housing around the turbine wheel. In this position the probe electrode represents one side of the running clearance gap and the tip surface of each passing turbine blade, at electrical ground potential is gainfully employed as an opposite capacitor electrode, and the other side of the running clearance gap. A change in the clearance gap is a change in the distance between capacitor electrodes and a change in electrical capacitance therebetween. Capacitance changes between the probe electrode and the passing blade tips are utilized to modulate an electrical oscillator signal and the modulated signal is processed to provide a further electrical signal indicative of the running clearance gap. In some gas turbine wheels the turbine blades in the circumferential row on the wheel have oppositely projecting shelf like segments at their free ends which meet or interfit with similar segments of an adjacent blade in the row to form a continuous circumferential rim or band surface encircling the blades and rotating therewith. Such a turbine wheel with an integral band or separately fitted band is referred to as a shroud ring turbine, and the shroud presents a continuous surface passing the probe as opposed to an unshrouded turbine wheel with blades having upstanding free ends which presents what may be described as an interrupted surface passing the probe electrode. A particularly advantageous clearanceometer will have the capability to sense variation in the running clearance between the continuous surface of a shroud ring and the encircling casing. Such a clearanceometer finds extended applicability to various other rotor members having continuous surfaces such as a sidewall or rim of a centrifugal compressor impeller.
As previously described, the probe member, and particularly the sensor electrode part thereof, is positioned in a very hostile environment of high temperatures in the presence of contaminating hot combustion gases from the combustion system of the engine, conditions which contribute to early probe deterioration resulting in, for example, a decrease in sensitivity and accuracy. As a consequence of the above noted factors, continuing efforts are expended to provide electrical capacitance probes which are more highly resistant to temperature extremes and contamination, and which not only have increased sensitivity, accuracy and stability but also wider applicability including sensitivity to continuous surfaces.